A basic laser includes a gain medium located between a pair of mirrors which define the laser resonator. One of the mirrors is a called the high reflector and reflects substantially all of the laser light. The other mirror is called the output coupler and is partially transmissive to the laser light.
The curvature of the resonator mirrors plays an important part in optimizing the performance of a laser. For example, the shape of the resonator mirrors effects the diameter and the mode of the laser beam. Typically, the mode of the laser beam is also partially controlled by inserting a mode control aperture within the resonant cavity. By properly adjusting the diameter of the laser beam in the cavity with respect to the aperture, a laser beam having single transverse mode or TEM.sub.00 mode characteristics can be generated.
FIG. 1a illustrates a plate 10 of the type which can be inserted into the cavity of a laser to control the mode of the beam. Plate 10 includes a mode control aperture 12. FIG. 1b illustrates a transverse mode profile 14 of a laser beam which could propagate in a laser without an aperture. Profile 14 corresponds to multiple transverse modes since it has a center section 16 of high intensity and two symmetric outer transverse lobes 18. A laser can be forced into single transverse or TEM.sub.00 mode operation if the size of the aperture is such that the outer transverse modes 18 experience high diffraction losses.
In order to optimize performance, the diameter D of the aperture 12 should be small enough so that the outer transverse modes of the beam experience enough diffraction losses such that they become extinguished. On the other hand, the diameter of the aperture 12 should be large enough so that the losses experienced by the TEM.sub.00 mode will be minimized thereby maximizing output power. The desired optimization can be achieved either by changing the size of the aperture or by changing the beam diameter. The diameter of the beam is controlled by the curvature of the resonator mirrors and their spacing.
Optimizing the relationship between the size of the mode control aperture 12 and the diameter of the beam is fairly straightforward when the laser is operating at a single wavelength or multiple wavelengths in a single narrow region. The problem becomes more difficult when the laser operates at multiple wavelengths which are not close together. The added complexity is due to the fact that the diameter of a beam is proportional to the square root of its wavelength. Thus, for a given resonator design having mirrors of a specific curvature, the diameter of the beam at the mode control aperture will be different for different wavelengths. Accordingly, it is not possible to optimize the mode control aperture for two disparate wavelengths beams.
In the past, various compromise solutions have been implemented to address the problem. For example, the diameter of the aperture can be selected to be an average between the ideal diameters for the different wavelength regions. Alternatively, the diameter of the aperture could be selected to optimize the mode control for one wavelength region while allowing the mode of the other wavelength region to suffer.
It would be desirable to provide a means for optimizing the mode performance of a laser which generates light in more than one wavelength region. In accordance with the subject invention, this means includes providing different wavelength selective coatings on the inner and outer surfaces of a single resonator mirror.
For many years, mirror coatings have been available which are designed to optimally reflect light in certain wavelength regions. For example, coatings have been developed to reflect either visible radiation or ultraviolet radiation. These type of coatings are often called cut-off coatings as they reflect all light either above or below a certain cut-off wavelength.
More recently there have been developed specialized coatings which are designed to reflect light at a single wavelength or in a very narrow wavelength region. These coatings are used on resonator mirrors to select wavelengths from lasers having gain mediums with multiple lasing transitions. As noted above, a single gain medium may be capable of lasing at multiple wavelengths. Each of these wavelengths has a different gain and potential power. The laser can be conditioned to laser at the selected wavelength by designing a coating which is reflective at the desired wavelength and transmissive at other wavelengths. By this arrangement, the nonselected wavelengths will have extremely high losses and will not laser and therefore the selected wavelength will be favored.
Information on selective coatings for mirrors can be found in the following documents, the disclosures of which are incorporated by reference. UK Patent Application GB 2,091,439, published July 28 1982, discloses a wavelength selective mirror for a CO.sub.2 laser. U.S. Pat. Nos. 4,615,033 and 4,615,034 both disclose coatings which allow the oscillation of the 488 nm line in an argon ion laser. All of the latter references disclose the application of the Wavelength selective coating on a single surface of a resonator mirror.
Accordingly, it is an object of the subject invention to provide a resonator mirror with wavelength selective coatings formed on both sides thereof.
It is another object of the subject invention to provide a resonator mirror with wavelength selective coatings formed on both sides thereof and wherein the curvature of the sides is different.
It is still a further object of the subject invention to provide a resonator mirror with wavelength selective coatings formed on both sides thereof and wherein one side is planar and the other side is curved.
It is still another object of the subject invention to optimize the performance of a multiwavelength laser.
It is still a further object of the subject invention to optimize the mode performance of a multiwavelength laser.
It is still another object of the subject invention to optimize the mode performance of a multiwavelength laser using a resonator mirror having wavelength selective coatings on the inner and outer surfaces thereof.
It is still a further object of the subject invention to provide a method of optimizing the mode performance of the laser by adjusting the angle and position of a resonator mirror having wavelength selective coatings on the inner and outer surfaces thereof and wherein one of the surfaces is planar and the other is curved.